disp: msm: dsi: modify handling of debugfs initialization

Add support to allow creation of debugfs node only if
CONFIG_DEBUG_FS is enabled.

Change-Id: I1ae2c4188a99e3ed88f59fc021efc01407bf942d
Signed-off-by: Harigovindan P <harigovi@codeaurora.org>
Signed-off-by: Satya Rama Aditya Pinapala <psraditya30@codeaurora.org>